The present invention pertains to the art of mechanical surface polishing of metals or metal workpieces particularly zinc die castings. The process described pertains to the fields known as barrel finishing and vibratory finishing in which workpieces are subjected to controlled agitation. In barrel finishing, barrels containing metal workpieces, abrasive media and a liquid are rotated at a designated speed. In vibratory finishing, machines are designed to vibrate metal workpieces, abrasive media and liquid at a designated frequency and amplitude. The constant agitation of the article to be deburred, polished or finished in the presence of an abrasive medium and liquid solution produces a scrubbing action that operates to finish the external areas of the workpiece uniformly and remove burrs. The present invention combines the mechanical process of barrel rotation or machine vibration with the chemical reaction between a metal and a dilute aliphatic acid solution to obtain a finished metal workpiece at low cost and in the shortest possible time.
The polishing of metals mechanically and electrolytically has been known for many years. In the electrolytic field, the combination of strong acids and high voltage creates mechanical and physical hazards and frequently result in considerable metal loss during processing. This has led to the desire for less hazardous polishing methods such as mechanical processes involving chemicals which do not react as strongly with metals as do the concentrated acids such as hydrofluoric, sulfuric, and hydrochloric acids. Barrel finishing for polishing metals is characteristically very slow, typically requiring from 6 to 12 hours. Furthermore, the impingement of workpieces against other workpieces generally results in indentations that impair the final surface finish. The use of vibratory processes for finishing metals is increasing primarily because of lower cost and elimination of the hazards resulting from the use of high voltage. Among the advantages of vibratory finishing processes are: (1) shorter processing times; (2) more uniform action over both recessed and exposed surface areas; (3) reduction of undesirable effects such as impingement or exposure of subsurface defects; (4) more effective use of processing capacity; (5) capability of finishing recesses, slots or holes; and (6) use of an apparatus that allows for easy inspection of the workpiece during operation. The present invention uses a new chemical accelerator solution in the conventional barrel and vibratory finishing processes to shorten the finishing time while still providing a smooth uniform surface for the castings.
It is an object of this invention to provide an economical one-step process for polishing metals using a chemical accelerator solution in a barrel or vibratory finishing process to obtain smooth smut-free surface finishes in the shortest possible time.
It is further an object of the invention to provide an accelerator solution which is readily disposable because it is biodegradable. This eliminates the need for chemical treatment of used solution prior to discarding it.
It is further an object of the invention to provide an accelerator solution in a pH range that allows for nonhazardous handling yet effectively accelerates the polishing process. Such a solution can be prepared from starting materials which can be characterized as stable solids capable of being weighed precisely and readily soluble in water.
Previous mechanical vibratory finishing and barrel finishing processes have used various liquid solutions to facilitate the finishing process, such as water, soap or detergent solutions, and dilute or concentrated inorganic acid solutions. Such processes frequently require several steps in which the abrasive media or the solution is replaced in successive stages. For example, a strong acid solution, used initially to remove extremely rough surfaces, must be replaced by a weak acid solution to obtain a uniformly smooth surface finish.
In an article published in the Journal Plating by Safranek and Miller, January, 1972, an accelerator solution consisting of sodium dichromate and sodium bisulfate is described. This sodium dichromate-sodium bisulfate solution presented certain practical difficulties which are eliminated in the present invention. Whereas the removal of hexavalent chromium from the rinse water waste is necessary in the prior process, the present accelerator solution does not require chemical treatment before disposing of the rinse water waste. Furthermore, dichromate is generally considered to be a passivating agent, that is, it possesses the property of changing a chemically active surface of a metal to a less reactive state. Finally, whereas in the earlier process, finishing required two one-hour steps using a different abrasive medium for each step, in the present invention, finishing may be accomplished in a single half-hour step usng only one abrasive medium.